A standard pressure-relief valve is connected between a high-pressure line and a sump and serves to bleed fluid from the line to the sump when pressure in the line exceeds a predetermined limit. This type of arrangement is used, for instance, when the pump is of the constant-displacement type and the load is of variable displacement, so the valve prevents dangerous overpressures from developing.
A standard such valve has a housing forming a chamber extending along an axis and a valve seat centered on the axis and subdividing the chamber into an outer high-pressure compartment and an inner low-pressure compartment. This seat defines the only fluid-communication path between the compartments. Conduits supply fluid under high pressure to the high-pressure compartment and withdraw fluid from the low-pressure compartment so that a pump and a load can be connected to the high-pressure compartment and a sump to the low-pressure compartment. A valve body centered on the axis is formed with a central radially outwardly open main groove, an outer face exposed in the high-pressure compartment, and a diametrally throughgoing control bore at the main groove. This body is axially displaceable through the seat between a closed position substantially blocking the seat with the main groove and control bore axially offset therefrom and an open position axially offset toward the low-pressure compartment from the closed position and with the main groove and control bore at the seat for flow through the main groove and seat between the compartments. A spring urges the valve body axially outward toward the high-pressure compartment into the closed position.
The relatively long stroke of such a valve body or spool between the open and closed positions causes the valve to respond somewhat slowly. With a great pressure differential the seat, which is normally cylindrical and fairly short, is wholly out of engagement with the spool, so that same can cant and jam. In addition with a standard spring the resistance to compression increases disproportionately with compression, so that inward movement of the valve spool is resisted with a force that increases as the displacement increases, so that in effect the pressure response can vary.